1. Field of the Invention
This invention relates to processes for the preparation of alkali metal dichromates and chromic acid by the electrolysis of monochromate and/or dichromate solutions in electrolytic cells in which the anode and cathode chambers are separated by cation exchanger membranes, an anolyte fluid containing dichromate and/or chromic acid being formed in the anode chamber and an alkaline catholyte fluid containing alkali metal ions being formed in the cathode chamber.
2. Description of Related Art
According to U.S. Pat. No. 3,305,463 and CA-A-739,447, the electrolytic preparation of alkali metal dichromates and chromic acid (CrO.sub.3) is carried out in electrolytic cells in which the electrode chambers are separated by a cation exchanger membrane.
Alkali metal dichromates are prepared by introducing alkali metal monochromate solutions or suspensions into the anode chamber of the cell where they are converted into an alkali metal dichromate solution by selective transfer of alkali metal ions into the cathode chamber through the membrane. For the preparation of chromic acid, alkali metal dichromate or alkali metal monochromate solutions or a mixture of alkali metal dichromate and monochromate solutions are introduced into the anode chamber and converted into solutions containing chromic acid. Sodium monochromate solutions and/or sodium dichromate solutions are generally used for these processes.
For the production of alkali metal dichromate crystals or chromic acid crystals, the solutions formed in the anode chambers of the cells are concentrated by evaporation: the crystallization of sodium dichromate, for example, may take place at 80.degree. C. and the crystallization of chromic acid at 60.degree.-100.degree. C. The crystallized products are separated off, optionally washed and dried.
In both processes, an alkaline catholyte fluid containing alkali metal ions is obtained in the cathode chamber. This catholyte fluid may consist of, for example, an aqueous sodium hydroxide solution or, as described in CA-A-739 447, of an aqueous solution containing sodium carbonate.
In the course of the process, deposits of compounds of polyvalent ions, in particular of alkaline earth compounds are formed, which impair the functional efficiency of the membrane within a short time until the membrane completely fails. The formation of these deposits is due to the presence of small amounts of polyvalent cations, in particular of calcium and strontium ions, in the alkali metal dichromate and/or alkali metal monochromate solutions used as electrolytes, such as those obtainable by industrial processes described in Ullmann's Encyclopedia of Industrial Chemistry, 5th Edition, Volume A 7, 1986, pages 67-97.
The object of this invention was to provide processes for the preparation of alkali metal dichromate and chromic acid by electrolysis which would be free from the disadvantages described above.